A stent is an elongated device used to support an intraluminal wall. In the case of a vascular stenosis, a stent provides an unobstructed conduit for blood in the area of the stenosis. An intraluminal prosthesis may comprise a stent that carries a prosthetic layer of graft material. Such a prosthesis may be used, for example, to treat a vascular aneurysm by removing the pressure on a weakened part of an artery so as to reduce the risk of rupture. Typically, an intraluminal stent or prosthesis is implanted in a blood vessel at the site of a stenosis or aneurysm endoluminally, i.e. by so-called “minimally invasive techniques” in which the stent, restrained in a radially compressed configuration by a sheath or catheter, is delivered by a stent deployment system or “introducer” to the site where it is required. The introducer may enter the body through the patient's skin, or by a “cut down” technique in which the entry blood vessel is exposed by minor surgical means. When the introducer has been threaded into the body lumen to the stent deployment location, the introducer is manipulated to cause the stent to be released from the surrounding sheath or catheter in which it is restrained (or alternatively the surrounding sheath or catheter is retracted from the stent), whereupon the stent expands to a predetermined diameter at the deployment location, and the introducer is withdrawn. Stents are typically expanded by spring elasticity, balloon expansion, or by the self-expansion of a thermally or stress-induced return of a memory material to a pre-conditioned expanded configuration.
Referring now to a stent deployment system of the prior art in FIG. 1, there is shown an endoluminal prosthesis 10 comprising a wire stent 12 affixed along its length to an outer graft cover 14, the graft and stent compressed inside outer sheath 16 (shown in cross-section). During the deployment process of endoluminal prosthesis 10 in a body lumen 20, such as a blood vessel, outer sheath 16 is retracted, and stent 12 expands against the walls 19 of the lumen 20 (shown in cross-section). During the expansion process, the partially-deployed, covered section 22 at distal end 23 and middle section 25 of integral stent/graft prosthesis 10 can block the flow of blood along arrow A temporarily until proximal end 24 is released from the sheath. As used herein, “proximal” is defined as meaning “closer to the end of the introducer remaining outside the body”, whereas “distal” is defined as meaning “farther from the end of the introducer remaining outside the body”. During deployment, the pressure of obstructed blood flow at covered section 22 may cause the prosthesis to migrate away from its intended location or become longitudinally compressed. If for some reason the deployment procedure becomes protracted, the blood flow blocked by covered section 22 may impart serious stress upon the patient. Thus, it is desirable to provide for unobstructed blood flow throughout the stent deployment process.
A construction known to the inventor prior to this invention comprises a device shown in FIG. 2 comprising stent 12′ and outer graft cover 14′ joined by a connection 30 to stent 12′ proximal the distal end 23 thereof. Prior to deployment, stent 12′ and graft liner 14′ are restrained in a compressed configuration by an outer sheath 16′ surrounding both the stent and the liner, and by an inner sheath 38 disposed between stent 12′ and liner 14′ proximally of connection 30. Deployment of this prosthesis is effected by first retracting outer sheath 16′, allowing distal portion of stent 12′ and then cover 14′ to fully expand independently. Stent 12′ is subsequently fully expanded proximal of the connection point by retracting inner sheath 38. During deployment of this device, blood flow can continue as indicated by arrows B.
The introducer construction having two sheaths as described above necessarily requires an introducer of somewhat larger diameter and lesser flexibility than most such introducers known in the art having only a single sheath.